In many applications where fluids are processed or stored, spherical tanks or cylindrical tanks having dished heads are utilized because of their relatively high pressure ratings compared to conventional, flat bottomed vessels of comparable wall thickness. However, spherical tanks and cylindrical tanks with dished heads have certain disadvantages. In applications such as ion exchange where the outlet is located above the tank bottom and is supplied by an underdrain across the tank, it has been necessary to support the underdrain with a barrier designed to withstand the full hydraulic pressure of the tank. This has been accomplished in some prior art vessels by installing a heavy steel plate across the diameter of the tank and supporting the plate on a layer of cement, or on a steel structure capable of withstanding the full internal tank pressure. In other prior art vessels a lightweight plate, commonly referred to as a false bottom, is installed across the diameter of the tank and an equalization leg is installed inside the tank to intermix the fluid above and below the false bottom to thereby equalize the pressure above and below the underdrain. In still other vessels the lower dished head is filled with a cured-in-place resin or with ion exchange resin.
The above-noted methods have proven to be generally unsatisfactory. Installation of a heavy steel plate and requisite supporting means across the internal cross-sectional area of the vessel increases the overall cost and weight of the vessel. Use of a cured-in-place resin may be prohibitively expensive, and the vapors emitted during the curing process may be toxic or explosive. The use of ion exchange resin below the underdrain of an ion exchange vessel or the use of a false bottom together with an internal equalization leg to intermix fluid above and below the false bottom also present disadvantages. Over an extended period of time ion exchange resin and the water treated in an ion exchange vessel are both corrosive to carbon steel vessels. The area of the vessel in which the ion exchange resin or water is present must, therefore, be lined with a corrosion resistant material, such as rubber. Thus, if ion exchange resin is used as a supporting means, or if water enters the dished area below the underdrain, the area below the underdrain also must be coated, which increases the cost and usually requires the addition of an extra manhole through which the coating operation is performed. Moreover, if an internal equalization leg is used, it must also be coated inside and out to protect against corrosion, thereby further increasing the overall cost of the vessel.
An object of the present invention is to provide a tank with underdrain which is relatively inexpensive to fabricate while still being corrosion resistant.
The present invention comprises installation of a fluid barrier between the tank outlet and the tank bottom to thereby form an upper volume above the barrier and a lower volume below the barrier. A substantially fluid-tight pressure equalization means communicates with the upper and lower volumes to equalize the pressure therebetween without a reduction of the total pressure in the tank. In one embodiment the pressure equalization means is located within the tank, while in another embodiment the equalization means is located external to the tank.